Recent developments in the engineering sciences have made the use of unmanned rotary winged aircraft (URWA) for commercial and military applications achievable, though these vehicles are just a fraction of the size of their manned counterparts. One of the major obstacles to UAV development, consequently, is designing around size and weight restrictions.
There are many different systems developed for counteracting rotor torque in rotary winged aircraft. Counter-rotating or co-axial rotors (counter rotating blades on a common center line) is one such system which has proven to be desirable for URWA application. There are two major advantages to using co-axial rotor systems. One is their relative compactness due to elimination of the tail rotor and the other is improved vehicle maneuverability as a consequence of each rotor being able to have its own collective and cyclic control. A co-axial rotor system may be responsive to six possible control options. These options are: both rotors increasing or decreasing collective pitch (lift control); one rotor increases while the other rotor decreases collective pitch (yaw control); both rotors have the same amount of longitudinal cyclic pitch (pitch control); differential longitudinal cyclic pitch between rotors (center of pressure control); both rotors have the same amount of lateral cyclic pitch (roll control); and differential lateral cyclic pitch between rotors (center of pressure control). Utilizing six control options presents a problem, though, because traditional helicopter flight controllers provide only four operator input parameters (pitch, roll, yaw, and collective). A conversion system to translate operator input into appropriate rotor blade control must, therefore, be provided.
Prior co-axial rotor URWAs utilize conversion systems comprised of mechanical mixing units which incorporate a complex series of linkages to provide push rod control outputs (corresponding to the aforementioned control options) for rotor blade displacement. Unfortunately, these units are large, complex, and heavy. Their complexity is disadvantageous to reliability and serviceability while their size and weight impose severe aircraft operational constraints and also necessitates that they be mounted on the airframe considerable distances from the rotors. Consequently, the aforementioned push rod controls must be long devices, further compounding system size, weight, and complexity problems. In addition to these drawbacks, mechanical systems are not easily adaptable if changes in gain, sensitivity, or phase are required or if additional control inputs or outputs are needed in the course of program development.